CL_CTMENU->ADD_FUNCTION method parameter missmatch

[thorsten-wolf-neptune]

CL_CTMENU method "add_function"'s parameters are currently typed as string.
That can lead to syntax errors because they should be a CHAR:
https://github.com/open-abap/open-abap-gui/blob/main/src/cl_ctmenu.clas.abap#L9-L10